1. Field of the Invention
The present invention relates to an image signal processing apparatus and a computer-readable recording medium recording an image signal processing program for performing an image signal process for a digitalized image signal from an image pickup device.
2. Description of the Related Art
Various image signal processes, one of which is a tone conversion process, are performed for an image signal.
If the image signal is a digitalized signal, the tone conversion process includes a process in which the number of bits varies between before and after the conversion, and a process in which the number of bits does not vary. Among the processes, for example, in the process in which the number of bits does not vary, if tone conversion is performed, a luminance region of which a tone width is extended, and a luminance region of which the tone width is compressed occur. Then, in the luminance region of which the tone width is compressed, a tone property is degraded. Particularly, whiteout occurs in a high luminance region, or blackout occurs in a low luminance region, which has been a problem in terms of image quality.
In response to such a problem, for example, Japanese Patent Application Laid-Open Publication No. 5-64075 describes a technique for obtaining a plurality of images with different exposure amounts with respect to the same shooting scene, and replacing a high luminance region in an image with a large exposure amount, with a corresponding region in an image with a small exposure amount, or a technique for replacing a low luminance region in the image with the small exposure amount, with a corresponding region in the image with the large exposure amount. In addition, usage of such techniques enables to suppress the occurrence of the region with the whiteout or the region with the blackout, and improve the image quality.
Moreover, for example, as an example of a space-variant tone conversion technique, Japanese Patent No. 3465226 describes a technique for analyzing a texture of an image, dividing the image into a plurality of regions based on a result of the analysis, and performing the tone conversion by using an independent tone characteristic for each region. Thereby, even in a scene with a large light-dark ratio, the region with the whiteout or the blackout can be suppressed well.
Incidentally, Japanese Patent Application Laid-Open Publication No. 2005-303802 describes a technique for estimating a noise amount, and performing a noise reduction process based on the estimated noise amount. The noise amount estimation technique described in the above described publication does not only estimate the noise amount based on a signal level, but also, further performs the estimation for each color signal based on an ISO sensitivity which dynamically varies for each shooting.